Electric vehicles are being put to practical use. It is known that some electric vehicles equip motors that have coils, through which high-frequency currents, such as 200 kHz, flow. Since, this type of motors consumes a lot of electric power, high-voltage large current needs to flow through the coils. However, the motors are driven by electricity supplied from batteries. Thus, there has been a need to reduce electric consumptions of motors. However, it is well known that loss of a high-frequency current is large while the current is flowing through a conducting wire because the current gathers around a surface of the conducting wire due to the skin effect. Therefore, the effective resistance of the conducting wire increases and the loss of electric power also increases. Worse thing is that the temperature of the motor become high such as 100 to 200° C. while the motor is running. Resistances of the traditional electric wires become undesirably high at such temperatures. Thus, higher voltage must be applied to the motor to generate the same mechanical force. This leads a large electric power consumption of the motor.
Litz wires have been commonly used to reduce the electric loss by the skin effect. The litz wire is constituted with bundled plural small-diametered wires, each of which is coated by an insulator. Thereby, the surface area of the litz wire is enhanced. However, since the litz wire is a bundle of the small wires, it is difficult to make the size and shape of the coils precisely homogeneous because the litz wire crumples up while being wound into the coil. Therefore, characteristics and performances of the coils made from the litz wire are not consistent. In addition, since it is difficult to wind the litz wire densely, it is difficult to make a coil from the litz wire that has high performance with a small size. Worsely, since each conducting wire constituting the litz wire has a small diameter, the litz wire is not suitable to apply a high-voltage large current. In order to make the litz wire capable of conducting a high-voltage large current, each conducting wire must have a large diameter. This leads the size of the motor to be large. This adds a weight to an electric vehicle and increases its electric power consumption.
To conquer such problems, there is a conducting wire that has grooves on its outer surface along the longitudinal direction (Japan published utility model application JP H05-15218). This wire has an increased surface area. When a high frequency current is applied to, this wire regulates the increase of effective resistance of the conducting wire caused by the skin effect. However, resistances of this wire still become large when the temperature becomes high. Thus, this wire is still not sufficient to reduce the electric consumption of the motors.
Recently cordless inductive power supply system has been getting popular to charge batteries of cellular phones. This system is also expected to be a future charging method for electric vehicles. This system enables to charge a battery without connecting a wire. The cordless inductive power supply system is composed of a transmitter and a receiver. To charge, a high-frequency high-voltage current is applied to the transmitter. When the receiver is close enough (but not in contact or wired), an electric power is transmitted to a receiver and a battery connected to the receiver is charged. To maximize the transmission efficiency, electric properties of electric wires such as impedance and inductance in the transmitter and receiver are critical. Currently, manufacturers produce the inductive power supply systems by their own format. Thus, in order to produce compatible transmitters and receivers, electric wires must be modified for each manufacturer. However, it costs a lot to develop electric wires for each manufacturers. Thus, an electric wire whose electric properties is easily attenuated is desired.